1. Field of the Invention
The present invention generally relates to lashing assemblies for securing stacks of containers to the decks of ships and, in particular, to tensioning mechanisms for providing a selected tension to a container lashing.
2. Description of Related Art
The shipping of containerized cargo has become commonplace in the maritime shipping industry. Typically, a large number of cargo containers is secured to the deck of a shipping vessel. Often, the containers are stacked in tiers of two or three or more on the deck of the ship.
The containers are secured to the deck of the ship by lashing cables or bars which hook into fittings rigidly mounted to the decks. Usually two or more lashing cables are used at each end of each stack to secure each stack of containers, with each container lashing mounting to the deck on opposing sides of the container and being locked to a corner fitting on the container. Tension is applied to the lashing cables to prevent movement of the container.
It is particularly important that proper tension be applied to the cable lashings. If tension is too great, the cables may damage the container, either by cutting into the container or by causing a portion of the container to buckle. If too little tension is provided, then the container may slide on the deck under heavy seas. Further, the container may rock or torsionally deform in some manner. In general, the container is subject to a wide range of forces including the pitch and heave of the ship, the yaw, sway, and surge of the ship, and the roll of the ship. Also, moving water and high winds at sea can impose large forces on the containers.
Given the various forces acting on the container at sea, it is desirable to be able to apply a particular tension for the lashing cables. Further, given that lashings may be untensioned by these forces, it is desirable to provide a means for quickly varying the tension of each of the on-board lashings while at sea.
However, conventional mechanisms for setting the tension of a cable lashing do not allow a preselected tension to be quickly and accurately achieved. For example, a conventional turnbuckle does not provide an adequate indication of the tension of the cables. Further, a conventional turnbuckle requires considerable time and strength to tighten. Commonly, two or more men are needed to set significant tension in each lashing using a conventional turnbuckle. Further, conventional turnbuckles include components, such as engaging screws and nuts, which deteriorate in the harsh environment of the deck of a shipping vessel so that they are extremely difficult to tighten effectively. These components often rust or corrode when exposed to the wet and briny environment of the deck of the ship. Once rusted or corroded, the turnbuckles cannot effectively be used.
In normal practice, lashing devices are lightly tightened in a cursory manner in an effort to keep them engaged so that they do not become undone in the progress of the voyage. The lashing devices are not tensioned to any great or consistent degree. There is an advantage to a lashing assembly that can be tightened consistently to a known tension that will endure for the duration of the voyage.
For these various reasons, it is difficult to quickly and effectively provide a desired tension to each of the many cable lashings used to secure numerous containers to a container ship. Hence, usually no attempt is made to set the cable lashings to an optimal, uniform, or substantial tension. Without optimal or uniform tension, the containers are subject to damage or destruction at sea. In some cases, the cable lashings themselves snap under the load, thus freeing the container, resulting in damage or destruction to the ship or to other containers on board the ship. Worse, the safety of the crew members is placed in jeopardy in the event one of the cable lashings snaps.